In many types of control circuits, and particularly in proportional temperature control circuits, it is necessary to provide a minimum time delay between the various stages of heating and/or cooling. In present day temperature control systems it has become common practice to provide a number of stages of heating and a number of stages of cooling so that the most efficient system can be developed. This is particularly true where electric heat has been used and a number of heating elements are progressively staged on and off. This is also true where cooling is involved. A number of compressors or cooling stages are staged on and off.
If no time delay function is provided in the staging of either the heating or the cooling equipment, any sudden change of the set point of the control thermostat can cause a number of stages to be called for at the same time. This same type of undesirable function also occurs whenever there is a momentary loss of power to the system and then the return of power. It can be readily understood that it is undesirable, if a number of stages of electric heat, or a number of cooling compressors are operating, to have them suddenly deenergized by a power failure and then all simultaneously reenergized when power is restored. This undesirable control action has been prevented by the use of some type of time delay element in the staging of the heating and cooling equipment. Most of the time delays currently being used are of the time constant type, where the amount of delay is proportional to the magnitude of the demand of the signal and is nonlinear as more and more stages are either turned on or off. Other types of time delays have been used that require a delay circuit for each of the heating or cooling stages. Where individual delay circuits are used for each stage, the delays can be made equal, but the expense of the circuitry makes their use prohibitive.